Various types of magnetic storage devices employ thin-film heads, such as disk drives and tape drives. The thin-film head is typically composed of one or more read elements and one or more write elements used to read/write information on the tape media, such as that described in U.S. Pat. No. 5,963,401 entitled “Magnetic tape head assembly including modules having a plurality of magneto-resistive head elements” by Richard Dee et al., which is hereby incorporated by reference as background material. Writing is performed by delivering a write signal to one of the write elements. The write signal creates a variable magnetic field at a gap portion of the write element. This magnetic field induces magnetic polarity transitions into the desired media track to effectuate writing of data on the media.
Reading of data from the media is performed by sensing the magnetic polarity transitions on the media as the media is moved across a thin-film head in a longitudinal direction. The magnetic polarity transitions on the media present a varying magnetic field to a read transducer in the head. The read transducer converts the varying magnetic field into an analog read signal that is delivered to a read channel for appropriate processing. The read channel converts this analog signal into digital signal(s) that are then processed by a computer system.
In thin-film heads having a plurality of transducer elements, magneto-resistive (MR) elements are typically used to read information from the media, due to their increased sensitivity during a read operation. The resistance of an MR element varies almost linearly with an applied magnetic field. During a read operation, the MR element is held very near (in the case of disk) or in contact with (in the case of tape) the media, to sense the varying magnetic transitions on a particular track. A constant DC current is passed though the MR element resulting in a variable voltage across the MR element due to its varying resistance. By Ohm's law (e.g. V=IR), the variable voltage is proportional to the varying resistance of the MR element, and hence is representative of the data stored on a particular track of the media. This variable voltage signal, which is the read analog signal, is then processed and converted to digital form for subsequent processing.
A simple MR head consists of a thin film of magneto-resistive material, such as Permalloy, between two insulating layers or shields. When the MR layer is formed, a magnetic field is typically applied in a direction parallel to the plane of the thin layer. Thus, the MR layer exhibits a uniaxial anisotropy with an easy-axis of magnetization parallel to the direction of the applied field. If an external magnetic field, such as from a magnetic tape, is applied normal to the easy-axis, the magnetization direction of the MR layer will rotate away from the easy-axis and towards the direction of the applied magnetic field. This magnetization rotation causes a change in resistance in the MR layer. When no external field is applied, the resistance is greatest. The resistance decreases with increasing applied field. For practical geometries of the MR layer, resistance as a function of applied field traces a bell-shaped curve. The MR element is often biased with an applied current such that a zero magnitude applied field results in a resistance near an inflection point on the resistance curve. Thus, small changes about a zero magnitude applied external field result in nearly linear changes in resistance.
There are many variables that can adversely affect the performance of a media drive such as a disk drive or tape drive. Temperature variations of the MR element are one such variable with adverse consequences. Because MR elements have positive temperature coefficients, increases in the temperature of an MR element can cause in increase in the resistance of the MR element. Similarly, decreases in the temperature of an MR element can cause a decrease in the resistance of the MR element. Since the read voltage signal is proportional to variations in resistance of the MR element multiplied by the constant bias current, whenever the temperature of the MR element is increased or decreased, a thermal signal is generated which adversely adds or subtracts to the value of the desired analog signal being read.
In thin film tape heads, there is a kind of noise that is sometimes referred to as contact noise. Contact noise occurs when a bump on the tape hits the MR element (or shield adjacent thereto) and momentarily cools the device. This cooling causes a momentary decrease in resistance of the MR element, and thereby produces a voltage spike in the output of the element. This spike, if it is large enough, can cause errors in reading the data by the data channel. The magnitude of this temperature fluctuation is proportional to the temperature rise of the shields and MR element. Therefore, one factor that produces excessive contact noise is excessive MR element/shield temperature.
Because both magnetic data signals and thermal signals cause variations in the resistance of the MR element, there is a need to develop a method and apparatus that mitigates these undesired thermal signals and their resulting effects. The present invention is designed to overcome the aforementioned problems. There is yet another reason for improving transducer thermal characteristics. As track widths are narrowed and read sensors thinned, current density becomes an issue. It is desirable to drive as much current as possible through the sensor to get the greatest amplitude—thus providing an improved signal-to-noise ratio signal read from the media. If the read sensor could cool better, more current can be applied. The present invention is also designed to enhance the signal being read from the media.